This invention relates to a novel method of preparing a coating composition and the coating composition prepared by that process. The composition comprises an epoxy resin and a functionalized hydrogenated elastomeric block copolymer. In another aspect, this invention relates to a coating composition comprising an epoxy resin, an epoxy adduct of a modified hydrogenated elastomeric block copolymer, and a curing agent.
Epoxy resins have long been utilized in coating compositions. Although cured epoxy resins are very hard, have excellent adhesion to most substrates and can form a very high gloss coating, they are generally rather brittle and easily crack under impact. Cured compositions based on high equivalent weight epoxy resins are significantly less brittle than cured compositions based on lower equivalent weight epoxy resins, but they are not easily handled or applied to workpieces. High equivalent weight epoxies are solids at room temperature and may be dissolved only in relatively dilute solutions. Additionally, solutions of high equivalent weight epoxies with greater than about 60% w solids contents are generally too viscous for application at room temperature. Use of these larger amounts of solvent is undesirable due to concerns of cost, environment, health and safety.
Another route to providing a less brittle epoxy coating is to add elastomeric materials to epoxy coatings and improve their flexibility via a rubber-toughening mechanism. Early attempts to accomplish this objective have included the incorporation in the epoxy of a low molecular weight liquid polymer which had been modified to incorporate reactive groups. Typical modified liquid polymers used to toughen epoxy based coatings are carboxyl terminated polybutadiene or carboxyl terminated butadiene-acrylonitrile copolymer. Molecular weights of about 5000 are usually preferred and, for example, a range from 2000 to 20,000 is taught by Gordon in U.S. Pat. No. 4,507,411. Higher molecular weight polybutadienes cause the epoxy resin composition to be too viscous. These low molecular weight polybutadienes are fairly soluble in epoxy resins before curing, but separate (precipitate) into an incompatible elastomeric phase upon curing of the epoxy resin. It is, then, difficult to control the size of the elastomer domains since it is difficult to control the rate of precipitation of the elastomeric phase. The resulting composition also exhibited a significantly lower softening temperature and a significantly lower glass transition temperature than the epoxy resin by itself.
Carboxyl terminated polybutadiene-epoxy compositions also exhibit poor thermal and oxidative stability due to the presence of unsaturated conjugated dienes.
The incorporation of carboxyl terminated butadiene-acrylonitrile copolymers instead of carboxyl terminated polybutadiene in epoxy compositions improves the compatibility of the elastomeric phase with the cured epoxy phase because of the more polar nature of the elastomeric phase. These compositions do not otherwise overcome the shortcomings of carboxyl terminated polybutadiene-epoxy resin compositions.
Heat curable molding compositions of epoxy resin toughened by acid functionalized selectively hydrogenated vinyl arene-conjugated diene block copolymers containing one or more blocks of polymerized conjugated diene and one or more blocks of polymerized vinyl arene are taught in Japanese Patent No. 58-17160. In the Japanese Patent '160, acid functionalized hydrogenated styrene-butadiene block copolymers which are solids at room temperature are incorporated into the epoxy resins without resulting in an excessively viscous composition prior to curing. Such compositions are disclosed as having excellent toughness, as measured by Izod impact strength, and adequate hardness and rigidity. The Japanese Patent specification discloses that the composition may be used as a coating, but there is no indication that a specific blending process is required to obtain an acceptable dried and cured coating from the composition. The examples of Japanese Pat. '160 are prepared by melt blending modified block copolymer which has been freeze-pulverized to a size of 80 mesh into an epoxy resin. Applicant has found that when a composition prepared in this manner is combined with hydrocarbon solvent and curing agent, dried and cured, the resultant coating had a very rough surface giving unacceptably low gloss, rendering the product nearly useless as a coating component.
A thermoplastic elastomer adhesive composition similar to the molding composition taught in Japanese Pat. '160 is taught in Japanese Pat. Application Publication No. 57-149369. The adhesive composition contains a functionalized hydrogenated monovinyl arene-conjugated diene block copolymer and an epoxy resin. A curing agent is not utilized because the epoxy groups act to crosslink the functionality of the block copolymer. The amount of epoxy resin in the adhesive composition is between 5 and 100 parts by weight for 100 parts by weight of functionalized block copolymer. Again, there is no indication that this composition would be useful as a coating composition, and no indication that a specific blending process is required to obtain an acceptable coating from such a blend. Additionally, because the epoxy component is present in only a minor amount, it acts as a crosslinking agent for the functionalized block copolymers, the epoxy curing agent is therefore not used and polyepoxide is never formed.
Hata, in Japanese Patent Application No. 62-143,912, discloses an epoxy composition containing a hydrogenated styrene-butadiene block copolymer. The block copolymer is present in an amount of 5 to 100 parts by weight for 100 parts by weight of epoxy resin. The block copolymer is claimed to be present as a dispersed phase with an average particle size of 0.05 to 10 microns. The particle size required is obtained by dissolving the block copolymer and the epoxy resin in a solvent, and then vaporizing the solvent at 60.degree. C. under a vacuum. Under these conditions, practically no chemical reaction occurs between the epoxy resin and the functionalized hydrogenated block copolymer. Although this composition, when cured, has excellent mechanical, chemical, and adhesive properties, and gives a much smoother coating than the composition of Japanese Pat. '160, cured coatings based on this composition still contain a substantial amount of grit causing less than acceptable gloss and reducing quality of appearance of the coating.
It is an object of the present invention to provide a process to produce an epoxy resin-modified hydrogenated block copolymer coating composition wherein the coating composition produced has good phase stability and low viscosity before curing, and when combined with a curing agent, applied to a workpiece and cured, has excellent gloss, hardness and flexibility. In another aspect it is an object to provide an epoxy resin-modified hydrogenated block copolymer coating composition which, after being cured, has excellent Sloss, hardness and flexibility, after being cured.